Mobile telecommunication terminals, such as mobile telephones or portable computers, are frequently used for the implementation of applications requiring precise location information. Thus, it is commonplace for these terminals to include means enabling them to estimate their position as accurately as possible. Satellite geo-location systems are particularly used. One example that can be cited is the GPS system, the acronym GPS standing for “Global Positioning System”. For a satellite geo-location system to function correctly, a sufficient number of satellites must be visible to the terminal. The choice of the number and the position of the satellites in the constellation of such a system makes it possible to satisfy this constraint. There are, nevertheless, situations in which the terminal cannot receive signals from a sufficient number of satellites. Such is, for example, the case in urban centers including large buildings preventing the satellites from being visible to the terminal or when the terminal is in partially or totally covered areas.
Other geo-location techniques can be used and rely notably on the characteristics of the waveforms and of the mobile radio system access networks such as GSM or UMTS, or else relying on wireless telecommunication systems such as Wifi or WiMax. However, the estimation accuracy is poorer than that obtained by the existing satellite geo-location techniques.
The single-frequency networks, also called SFN networks, are more rarely used in the geo-location context. An SFN network is a network comprising a plurality of transmitters and receivers. In this type of network, each transmitter transmits the same signal in a synchronized manner and on one and the same carrier frequency. In the absence of multiple paths, a receiver receives several replicas of the same signal originating from a number of transmitters. The various replicas are assigned an attenuation and a delay both of which are different, said attenuation and said delay depending notably on the distance between the receiving terminal and each of the transmitters. These characteristics of the SFN networks make it difficult for a receiving terminal to identify the origin of a replica of the signal, that is to say, to identify the transmitter. Locating the terminal by using the signals transmitted by the transmitters of a network is in principle possible provided it is possible to locate and identify said transmitters.
Digital tattooing is a known technique which notably enables the terminals to identify the transmitters of an SFN network. This technique is often referred to as “watermarking”. The principle of this technique is that a signal identifying the transmitter is superposed on the signal containing the application data intended for the user of the receiving terminal. The signal including these user data is hereinafter in the description called the useful signal. Said user data correspond, for example, to video streams associated with television channels. The identification signal is usually superposed on the useful signal with a very low relative power, which does not disturb the reception of the user data. However, the identification signal is scrambled when approaching another transmitter. This corresponds to the phenomenon that is well known to those skilled in the art called “near-far effect”. In this case, only the nearest transmitters can be identified. As explained previously, to estimate the position of a terminal, it is necessary to identify as many transmitters as possible.